Biology of Thermoregulation: Endotherms

Biology of Thermoregulation

• Thermoregulation is the process of maintaining a constant internal body temperature, regardless of the ambient environmental temperature.

Endotherms (Warm-Blooded)

• Maintain a constant internal body temperature, regardless of the ambient environmental temperature.
• Require a lot of food to fuel their high metabolism.
• Minimize heat exchange with the external environment through insulating fur or feathers.
• Can maintain a constant internal temperature, e.g., humans, dolphins, bears, leopards.

Ectotherms (Cold-Blooded)

• Internal temperature varies with the ambient environmental temperature.
• Depend on heat exchange with the external environment.
• Require relatively little food.
• Examples include fish, frogs, and some reptiles.

Thermoregulation Methods

• Insulation (fur, feathers, blubber, coloration)
• Metabolic heat production (physiological thermoregulation)
• Counter-current heat exchange (arteries and veins)
• Behavioral thermoregulation (posture, orientation, and microclimate selection)

Behavioral Thermoregulation

• Examples: lizard wanting to increase temperature will 'spread eagle' on a hot rock (posture) and turn its back to the sun (orientation).
• Examples: bats, hummingbirds, and small marsupials use torpor (mini-hibernation) to reduce metabolic activity and body temperature.

Hibernation and Estivation

• Hibernation: long-term torpor (can be 6 months) in the winter months to conserve energy.
• Estivation: long-term torpor (can be 6 months) in the summer months to avoid damage from high temperatures.

Timing of Activities

• Nocturnal: active at night (e.g., owl, mice, koala)
• Diurnal: active during daytime (e.g., 'grazers', gazelles, elephants)
• Crepuscular: active at dawn/dusk (e.g., deer, rabbits, cats, red pandas, most birds)
• Cathemeral: active at periods throughout 24 hours (e.g., some lemurs)

Cooling Off Techniques

• Rolling or wallowing in mud
• Taking a 'dip' or standing in water
• Going underground or using caves
• Lying in shade
• Flying in high altitudes

Thermoregulation Pathologies

• Hyperthermia: body temperature too high, fever
• Hypothermia: body temperature too low, metabolism slows

Homeostatic Balancing of Body Temperature

• Peripheral and body core receptors sense temperature changes
• Hypothalamic thermoregulatory center integrates and initiates shivering, non-shivering thermogenesis, and vasoconstriction.
• Metabolic heat production is usually required to maintain balance.
• Balance is maintained in a narrow range, usually higher than the environment.
• Thermoneutral zone: represents ambient conditions where heat gain equals heat loss (= thermal comfort; 28-31°C in naked humans).